Today ordinary chewing gums and bubble gums generally utilize as their gum base one or a combination of two or more natural or synthetic elastomers. The gum base that is selected provides the chewing gum with its masticatory properties. A chewing gum base is normally admixed with sugars or synthetic sweeteners, perfumes, flavors, plasticizers, and fillers; and then milled and formed into sticks, sheets, or pellets. Cottonseed oil is sometimes also added to give the gum softness. Styrene butadiene rubber (SBR) is a synthetic elastomer that is widely used as a gum base in chewing gums. However, SBR is not widely used in manufacturing soft chew gums because it lacks the desired physical properties. Polyisobutylene is widely used in manufacturing soft chew gums even though it is much more expensive than SBR.
In any case, chewing gum compositions are typically comprised of a water soluble bulk portion, a water insoluble chewing gum base portion and typically water insoluble flavoring agents. The water soluble portion dissipates with a portion of the flavoring agent over a period of time during chewing. The gum base portion is retained in the mouth throughout the chewing process.
The gum base includes a number of ingredients that are subject to deterioration through oxidation during storage. The insoluble gum base generally comprises elastomers, elastomer plasticizers, waxes, fats, oils, softeners, emulsifiers, fillers, texturizers and miscellaneous ingredients, such as antioxidants, preservatives, colorants and whiteners. The compounds contain carbon-carbon double bonds, such as fats, oils, unsaturated elastomers and elastomer plasticizers, are susceptible to oxidation. The gum base constitutes between 5-95% by weight of the chewing gum composition, more typically 10-50% by weight of the chewing gum, and more commonly 15-35% by weight of the chewing gum.
Commonly used natural or artificial antioxidants/preservatives include beta-carotenes, acidulants (e.g. Vitamin C), propyl gallate, butylated hydroxyanisole (BHA), and butylated hydroxytoluene (BHT). BHA and BHT are synthetic antioxidants that most commonly used stabilize chewing gum base.
U.S. Pat. No. 4,489,099 discloses the use of Vitamin E in combination with dilauryl thiodipropionate (DLTDP), as a stabilizer for a styrene-butadiene rubber in chewing gum. U.S. Pat. No. 5,132,121, U.S. Pat. No. 5,200,213, and U.S. Pat. No. 5,270,060 disclose a use of 0.01-1.00% by weight of a tocopherol mixture comprising 7-20% by weight alpha tocopherol, 45-75% by weight gamma tocopherol and 18-32% by weight delta tocopherol to stabilize chewing gum base.
Chewing gum made from highly saturated polymers will not degrade under typical outdoor conditions and remains soft and sticky on sidewalks. Typical chewed stick gum will therefore strongly adhere to sidewalks and resists standard cleaning and removal efforts. When chewed gum is spit out onto sidewalks it picks up dirt, turns black and becomes an ugly form of litter. The chewed gum will also stick to other surfaces, such as shoe soles and clothing, which is makes it a very annoying nuisance. Gum removal from sidewalks, in particular, has become such serious issue that in Singapore, the sale and possession of chewing gum is actually illegal. In other cities around the world, the buildup of this unsightly black gum on sidewalks creates an eyesore.
There has been a long felt need in the chewing gum industry for a styrene-butadiene rubber (SBR) that has the physical properties needed for utilization in manufacturing chewing gum having soft chew characteristics. This is because it is much less expensive than other elastomers, such as polyisobutylene rubber, that are normally used in such applications. However, conventional SBR does not have the soft and smooth chew characteristics needed in soft chew applications. Furthermore, conventional SBR typically also has undesirable odor and taste characteristics that have generally limited its use in chewing gum base formulations.
The present invention relates to a technique for synthesizing SBR that can be used in manufacturing chewing gum base for soft chew applications. This technique also improves the taste and odor of the SBR which makes it generally more desirable for use in all types of chewing gum. The technique used in this invention to reduce taste and odor involves copolymerizing about 1 phm to about 30 phm styrene and about 70 phm to about 99 phm of 1,3-butadiene in an aqueous emulsion, wherein said copolymerization is conducted at a temperature which is within the range of about 1xc2x0 C. to about 70xc2x0 C., and wherein said copolymerization is initiated with an initiator system which is comprised of (a) a free radical generator, (b) a reducing agent selected from the group consisting of ascorbic acid, isoascorbic acid, and ascorbic acid derivatives having the structural formula: 
wherein R is an alkyl group containing from 1 to 30 carbon atoms, and (c) a water-soluble metal salt of iron, copper, cobalt, nickel, tin, titanium, vanadium, manganese, chromium or silver. It is important for this process to be conducted without utilizing sulfur containing initiator systems.
The present invention also specifically discloses a process for synthesizing styrene-butadiene rubber that is particularly useful in manufacturing chewing gum base for soft chew applications which comprises copolymerizing about 1 phm to about 12 phm styrene and about 88 phm to about 99 phm of 1,3-butadiene in an aqueous emulsion, wherein said copolymerization is conducted at a temperature which is within the range of about 1xc2x0 C. to about 20xc2x0 C., and wherein said copolymerization is initiated with an initiator system which is comprised of (a) a free radical generator, (b) a reducing agent selected from the group consisting of ascorbic acid, isoascorbic acid, and ascorbic acid derivatives having the structural formula: 
wherein R is an alkyl group containing from 1 to 30 carbon atoms, and (c) a water-soluble metal salt of iron, copper, cobalt, nickel, tin, titanium, vanadium, manganese, chromium or silver.
The present invention further discloses a chewing gum base, comprising: (1) about 5 weight percent to about 95 weight percent styrene-butadiene rubber, wherein said styrene-butadiene rubber has a bound styrene content of about 1 weight percent to about 10 weight percent, and wherein said styrene-butadiene rubber has a RPA t80 of at least 0.009 minutes; (2) about 0 weight percent to about 75 weight percent of an elastomer plasticizer selected from the group consisting of natural rosin esters and synthetic terpene resins; (3) about 1 weight percent to about 65 weight percent of a filler material; and (4) a gum base stabilizer.
The present invention also reveals a chewing gum which comprises: (1) about 5 weight percent to about 95 weight percent styrene-butadiene rubber, wherein said styrene-butadiene rubber has a bound styrene content of about 1 weight percent to about 10 weight percent, and wherein said styrene-butadiene rubber has a RPA t80 of at least 0.009 minutes; (2) about 0 weight percent to about 75 weight percent of an elastomer plasticizer selected from the group consisting of natural rosin esters and synthetic terpene resins; (3) about 1 weight percent to about 65 weight percent of a filler material; and (4) a gum base stabilizer, (5) a sweetener, and (6) a flavor. It is highly desirable for the gum base and the chewing gum to be substantially free of sulfur containing compounds.